In a redox flow battery (RF battery) described in PTL 1, inner sealing grooves and outer sealing grooves are formed in both surfaces of a frame (frame body) at positions where the inner sealing grooves and the outer sealing grooves respectively face each other, and inner seals and outer seals are respectively disposed in the inner sealing grooves and the outer sealing grooves. O-rings are used as the inner seals and outer seals. By pressing a stack of cell frames by fastening end plates that sandwich the stack of cell frames from both sides thereof, the inner seals and the outer seals press-contact a membrane, and thereby leakage of an electrolyte of a cell from a space between the frames is prevented.